DENIERS SAY:

The climate has changed over and over, long before humans had anything to do with it.

SCIENCE SAYS:

Climate change doesn't just happen — it happens for a reason. And today, the reason is us.

Has our climate changed before? Absolutely! Nobody disagrees with that. But arguing that humans aren’t the cause of climate change today is like arguing humans can’t cause forest fires because they've been started by natural causes in the past. Basic physics tells us something important: Climate change happens for a reason. Some past changes in the climate were driven by the sun burning brighter, or by an increase in volcanic activity. That’s not the case now. There is overwhelming scientific evidence that humans are causing most of today’s climate change by burning dirty fossil fuels and sending carbon pollution into the atmosphere at an unprecedented rate.

Additional info from Skeptical Science

A common skeptic argument is that the climate has changed naturally in the past, long before SUVs and coal-fired power plants, so therefore humans cannot be causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion. To understand this, first you have to ask why climate has changed in the past. It doesn't happen by magic. Climate changes when it’s forced to change. When our planet suffers an energy imbalance and gains or loses heat, global temperature changes.

There are a number of different forces which can influence the Earth’s climate. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, they emit particles into the atmosphere which reflect sunlight, and the planet cools. When there are more greenhouse gases in the atmosphere, the planet warms. These effects are referred to as external forcings because by changing the planet's energy balance, they force the climate to change.

It is obviously true that past climate change was caused by natural forcings. However, to argue that this means we can’t cause climate change is like arguing that humans can’t start bushfires because in the past they’ve happened naturally. Greenhouse gas increases have caused climate change many times in Earth’s history, and we are now adding greenhouse gases to the atmosphere at an increasingly rapid rate.

Looking at the past gives us insight into how our climate responds to external forcings. Using ice cores, for instance, we can work out the degree of past temperature change, the level of solar activity, and the amount of greenhouse gases and volcanic dust in the atmosphere. From this, we can determine how temperature has changed due to past energy imbalances. What we have found, looking at many different periods and timescales in Earth's history, is that when the Earth gains heat, positive feedbacks amplify the warming. This is why we've experienced such dramatic changes in temperature in the past. Our climate is highly sensitive to changes in heat. We can even quantify this: When you include positive feedbacks, a doubling of CO2 causes a warming of around 3°C.

What does that mean for today? Rising greenhouse gas levels are an external forcing, which has caused climate changes many times in Earth's history. They're causing an energy imbalance and the planet is building up heat. From Earth's history, we know that positive feedbacks will amplify the greenhouse warming. So past climate change doesn't tell us that humans can't influence climate; on the contrary, it tells us that climate is highly sensitive to the greenhouse warming we're now causing.